An example of such power supply apparatus includes an input-side AC-to-DC converter which converts a commercial AC voltage to a DC voltage. The DC voltage is applied to an inverter where it is converted to a high-frequency voltage, which, in turn, is applied to a voltage transformer. The voltage transformer transforms the high-frequency voltage to a high-frequency voltage having a predetermined value. The voltage-transformed high-frequency voltage from the transformer is converted back to a DC voltage in an output-side high-frequency-to-DC converter. The resulting DC voltage is applied to a load.
The input-side AC-to-DC converter and the output-side high-frequency-to-DC converter each include at least one diode. The inverter includes at least one semiconductor switching device, e.g. an IGBT.
Since the described power supply apparatus includes an inverter, it can use small-sized reactors in the transformer and the output-side high-frequency-to-DC converter, which enables the downsizing of the power supply apparatus.
The power supply apparatus is placed in a casing, and, therefore, Joule heat generated by some components tends to accumulate in the casing. The diodes used in the input-side and output-side AC-to-DC converters and the power semiconductor device used in the inverter, in particular, generate a large amount of heat. A fan is disposed in the casing for the purpose of forcibly cooling such devices.
When this power supply apparatus is used with, for example, a welder or a cutter, of which the load includes a torch and a workpiece, it is used for a plurality of operation cycles. Each cycle consists of one running period in which the power supply apparatus operates in a running mode to generate an arc between the torch and the workpiece, and one pausing period in which the apparatus operates in a pausing mode. In the pausing mode, the apparatus is temporarily stopped. The time period of one cycle is predetermined for a particular apparatus. For example, it may be 10 minutes. Also, the proportion of the period of the running mode in one cycle (i.e. running ratio) is predetermined on the basis of the rating of the particular power supply apparatus.
Although the fan is driven to rotate in both the running and pausing periods, the power semiconductor devices including the above-mentioned diodes and semiconductor switching device, generate heat only during the running periods, causing the temperature in the casing to rise. During the pausing periods, the temperature within the casing rapidly decreases because the generation of heat by the power semiconductor devices is stopped and the fan is operating to cool the interior of the casing.
Experiments have revealed that the lifetime of power semiconductor devices decreases as the difference .DELTA.Tc between the highest temperature and the lowest temperature during their operation increases. Temperature difference frequently occurs in a power supply apparatus for an arc welder or an arc cutter, in particular, which has a predetermined running ratio, which results in decrease of the lifetime of power semiconductor devices used therein. In addition, the power consumption of such power supply apparatus is large because the fan continuously operates even during a time interval between one welding operation and another or between one cutting operation and another, during which the fan need not be operated.
There is a power supply apparatus in which the temperature within the casing is measured, and the rotation of the fan is controlled in accordance with the measured temperature, so that unnecessary power consumption can be reduced. However, this arrangement cannot provide a solution to the reduction of the lifetime of diodes and semiconductor switching devices used in the apparatus.
The present invention is based on a discovery that power semiconductor devices can be used for a longer time as the difference between the highest and lowest temperatures to which they are subjected during operation is smaller. An object of the present invention is to prolong the lifetime of power semiconductor devices used in a power supply apparatus and reduce unnecessary power consumption of the apparatus.